The composition, density (i.e., the weight) and the like of LNG differs depending on the location of its production. In recent years, as the demand for LNG has increased, the development of different-density LNG storage technology for storing a plurality of types of LNG that each have a different density in the same LNG tank has been promoted. This different-density LNG storage technology has the considerable economic merits that it facilitates the handling and distribution of LNG, and it enables costs to be reduced. On the other hand, when a mixture of LNG of different densities is being stored, it is necessary to formulate measures to counter rollover which occurs as a result of stratification inside the LNG tank.
‘Stratification’ refers to the formation of a plurality of liquid layers of mutually different densities that occurs if a plurality of types of LNG that each have a different density are introduced into an LNG tank when LNG having a high density (i.e., heavy LNG) collects on the bottom of the LNG tank and LNG having a low density (i.e., light LNG) accumulates in the top thereof. ‘Rollover’ refers to a phenomenon in which, when the difference in densities between vertically adjacent layers within an LNG tank that has become stratified in the manner described above decreases due to the introduction of heat from the outside so that the boundary between these layers disappears, the heat energy that had accumulated up to that point in the lower layer is released in a short space of time from the liquid surface in the form of the generation of an immense quantity of BOG (Boil Off Gas).
If the quantity of BOG that is generated by this rollover exceeds the processing capability of the BOG compressor, then in order to control the rise in the pressure inside the tank, it is necessary to operate a safety valve so as to discharge the excess BOG to the outside of the tank. However, if the quantity of BOG that is generated exceeds even the excess BOG discharge capability of the safety valve, then it is no longer possible to control the rise in pressure inside the tank, and there is a possibility that the tank itself may rupture. Accordingly, in order to avoid the occurrence of rollover, it is necessary to suppress stratification inside the LNG tank as much as possible.
Conventionally, two receiving pipes are provided that penetrate the roof of an LNG tank, and a leader pipe that extends as far as the bottom portion of the LNG tank is provided under one of the receiving pipes so that heavy LNG is received from the top portion of the tank through the receiving pipe. In contrast, light LNG is received from the bottom portion of the tank through the receiving pipe and the leader pipe. As a result, the mixing together of different-density LNG is promoted and stratification is suppressed.
Note that an LNG receiving structure of a conventional LNG tank is described in Patent documents 1 and 2 (see below).